This invention relates to the field of semiconductor processing. More particularly the invention relates to a method for forming a silicon germanium complimentary metal oxide semiconductor channel in a semiconductor device.
As integrated circuits become more complex, engineers and scientists look for ways to reduce the surface area required on the substrate for the integrated circuit. Reducing the size of the integrated circuit tends hold to the chip size to a reasonable level, and the device also tends to be operable at a higher speed. Unfortunately, as the size of integrated circuits is reduced, limits and constraints in regard to the proper operation of various device structures are encountered.
For example, one of the fundamental challenges for developing logic circuits is optimizing the speed of the logic device. Because the mobility of holes tends to be lower than the mobility of electrons in many semiconducting materials, the drive current of PMOS transistors tends to be commensurately lower than the drive current of a similarly sized NMOS device, when each is driven at equal supply voltage potentials. Thus, the low drive current of the PMOS device tends to be one speed limiting parameter in certain circuits, such as a logic inverter.
One method of compensating for this situation is to form larger PMOS devices, relative to the size of the NMOS devices. Thus, the larger PMOS structure has an ability to carry a commensurately larger drive current at the same supply voltage potential, because of the increased numbers of carriers. Unfortunately, addressing the problem by increasing the size of the PMOS structures is in direct opposition to the design goal of creating ever smaller integrated circuits.
Another method of compensating for the speed difference is to increase the drive current of the PMOS device by increasing the potential of the supply voltage at which it is driven, relative to that of the NMOS device. Unfortunately, it is often desirable to drive both the PMOS device and the NMOS device at the same potential. Thus, compensating for the difference in drive currents between the two structures in this manner is somewhat unsatisfactory. Further, if the higher drive potential is available on the chip, then it seems somewhat of a waste to not use it to drive the NMOS device at an even greater drive current. Therefore, providing different supply voltages to the different devices to balance the drive currents of the devices tends to be somewhat of an inelegant solution. Additionally, increasing the supply voltages may jeopardize the reliability of the devices due to hot carrier injection into the gate dielectric.
What is needed, therefore, is a structure, and a method for its formation, that can be used in PMOS and NMOS devices to achieve high drive currents and keep leakage currents low.
The above and other needs are met by a method for fabricating a semiconducting device on a substrate, where the improvement includes forming a strained silicon germanium channel layer on the substrate. A gate insulation layer is formed on top of the strained silicon germanium channel layer, the gate insulation layer preferably formed at a temperature that does not exceed about eight hundred centigrade. A gate electrode is formed on top of the gate insulation layer, and the gate electrode is patterned. A low dose drain dopant is impregnated into the substrate, and activated with a first laser anneal. A source-drain dopant is impregnated into the substrate, and activated with a second laser anneal. The source drain impregnations may be preceded by a high dose preamorphization implant. This preamorphization implant allows the fabrication of ultra shallow and highly activated source drain junctions by lowering the melting temperature of the silicon layer containing the dopant.
Thus, by use of the strained silicon germanium channel layer, the mobility of electrons and holes is increased by up to about seventy-five percent when applied to a 100 nanometer technology. However, use of strained silicon germanium imposes strong limitations in the applied thermal budget once the strained silicon germanium layer is formed. If the strained silicon germanium layer is annealed at a temperature higher than about 850 centigrade, then the layer tends to relax and loose it favorable conduction properties. Therefore, the subsequent layers are formed at temperatures that are preferably below about eight hundred centigrade. Further, laser annealing is used for locally activating both the low dose drain dopant and the source-drain dopant. Because the strained silicon germanium layer tends to have a lower thermal conductivity than that of the substrate, the heat generated by the laser pulses does not tend to anneal the strained silicon germanium channel layer during the laser annealing. Annealing by other more traditional methods, such as rapid thermal annealing or furnace annealing, would tend to exceed the thermal budget of the strained silicon germanium channel layer.
In various preferred embodiments the strained silicon germanium channel layer is formed such as by deposition with a chemical vapor deposition process or growth with a surface reaction epitaxy process. The strained silicon germanium channel layer further is formed to a thickness of preferably about thirty angstroms, with a tensile strain and a composition of silicon of about seventy percent. Further, the strained silicon germanium channel layer for an NMOS device is preferably formed with an in situ boron dopant at a concentration of about 1017 atoms per cubic centimeter. Still further, an additional dopant may be implanted into the strained silicon germanium channel layer after it is formed. PMOS devices are preferably formed with dopants compatible with PMOS designs, and with concentrations commensurate with such designs.
In a most preferred embodiment, after the step of activating the low dose drain dopant with the first laser anneal, an insulating layer is formed around the gate electrode, at a temperature that does not exceed about eight hundred centigrade, and a spacer is formed around the gate electrode. The spacer is formed of a material that is reflective to the second laser anneal. Thus, standard materials for the spacer, such as silicon oxide or silicon nitride are not preferred for this application, because they tend to be transparent to the laser beam emissions. However, depending on the wavelength of the applied laser, silicon oxide or silicon nitride may still be used as a spacer material as long as the laser beam is reflected and protects the area underneath.
Most preferably the insulating layer around the gate electrode is formed of silicon oxide and the spacer material is formed of polysilicon. In other preferred embodiments, the substrate is silicon, the gate insulation layer is silicon oxide, and the gate electrode is amorphous silicon.
According to another aspect of the invention, a semiconductor device is presented, the improvement being a strained silicon germanium channel layer and a spacer around the gate electrode formed of a material that is reflective to the laser anneal.